Structure and electrochemical behaviour of anodically deposited Co-Mn mixed oxide thin films for supercapacitor applications

The current technology of supercapacitors is based on electrochemical double layer capacitors with electrodes made of carbonaceous materials. Although their charge storage mechanism based on the EDL concept is what confers them their high power, excellent reversibility and long life time, hence distinguishing them from batteries, their low specific energy and capacitance is a major drawback that limits the range of applications to those devices requiring low amounts of energy. With the goal of improving the latter two parameters to a level comparable or competitive to typical SLI (starting, lighting, ignition) lead-acid batteries having energy density up to about 40 Wh/kg (at long run times), research has moved onto those electrochemically active materials characterized by fast and reversible redox reactions that base their charge storage mechanism on the concept of pseudocapacitance. Amongst these materials, transition metal oxides like RuO2 have raised particular interest for the high specific energy and capacitance demonstrated thanks to a wider electrochemical stability window, along with the high reversibility of the charge discharge reactions taking place that guaranteed a long lifetime. Unfortunately, the high cost and toxicity of RuO2 has limited their use to military applications and a more economic and environmentally friendly alternative has been sought after and found in manganese oxide. Notwithstanding the high theoretical capacitance of manganese oxide, the low conductivity and still limited capacitance have prevented this oxide to finally introduce pseudocapacitors to the global market scenery. The initial goal of this thesis was to investigate a Co-Mn mixed oxide with spinel microstructure and stoichiometric formula of CoMn2O4 deposited onto a steel substrate via anodic potentiostatic electrodeposition at 1.2V, and evaluate the morphology and electrochemical performance on an attempt to improve the specific capacitance of pure MnO2. Unfortunately, the spinel microstructure that theoretically could have constituted a suitable material as a supercapacitor electrode, experimentally resulted having various cracks and an extremely poor specific capacitance that was even lower than the pure MnO2. Having observed the poor electrochemical performance of the mixed Co-Mn oxide with X= Co/(Co+Mn)= 0.33, the structure and capacitive behaviour of oxides with lower values of the X ratio were investigated through cyclic voltammetry, charge/discharge curves, impedance spectroscopy and long term cycling at a current density of 10A/g for 5000 cycles. The outcome of the experiments ultimately showed that, after 5000 cycles at 10A/g, the anodically deposited oxides with X < 0,15 had a specific capacitance improvement of approximately 16% as compared to the pure MnO2 proving that mixed oxides, in particular the addition of Co to manganese oxide, may be beneficial for the enhancement of the electrochemical performance of these electrodes.